1. Field of the Invention
This invention relates to a method of blending ethylene vinylacetate copolymers and polydiorganosiloxane gums and to the thermoplastic blends produced.
2. Description of Prior Art
Many useful materials have been obtained by blending polydiorganosiloxanes and polyethylenes to achieve new combinations of properties. Since the polymers are generally incompatible, it is often difficult to obtain stable, useful blends. Consequently, techniques such as block or graft copolymerization may be necessary to hold the polymers together in a useful state.
Safford in U.S. Pat. No. 2,888,419 and Precopio et al. in U.S. Pat. No. 2,888,424 teach that polyethylene, filler and organopolysiloxane, which may be highly viscous masses of gummy elastic solids, can be intimately mixed at temperatures up to 135.degree. C. to provide a more extrudible composition with less nerve and improved aging characteristics. They teach that the filler acts as a blending aid for these two polymers which results in a homogeneous product which is difficult to otherwise obtain. Vostovich et al. in U.S. Pat. No. 2,930,083 teach that improved processing of gelled or crosslinked polyethylene can be obtained by mixing gelled polyethylene and organopolysiloxanes which are highly viscous masses to gummy elastic solids on a mill or Banbury mixer at 220.degree. F. for four minutes.
Safford in U.S. Pat. No. 3,227,777 teaches vulcanizing ethylene-propylene copolymers with an alkenylpolysiloxane and a bis(aralkyl) peroxide. Safford mills the copolymer of ethylene and propylene with the alkenylpolysiloxane and peroxide and thereafter vulcanizes the resulting mixture at 100.degree. C. to 175.degree. C. The properties of good heat resistance, electrical properties are due to the cured state of the ethylene-propylene copolymers.
Skiens et al. in U.S. Pat. No. 3,798,185 teaches the incorporation of polyorganosiloxanes in certain thermoplastics which are compatible at elevated melt temperatures and which have an oxygen permeability constant of at least about 0.5.times.10.sup.-10 cc. cm/cm.sup.2 sec cm Hg. The uniform melt is extruded as a blend to produce a membrane with improved gas permeability. Skiens further teaches that useful thermoplastics include ethylene vinylacetate copolymers, but that poly-alpha-olefins such as poly-4-methylpentene-1 are preferred. Skiens also teaches that the polydiorganosiloxane liquids used may be quite viscous, but siloxanes having a low viscosity at ambient temperature are preferred.
British Pat. No. 1,294,986 teaches incorporating silicone fluid with a viscosity not greater than 100 centistokes at 25.degree. C. in ethylene or ethylene copolymers such as ethylene vinylacetate copolymers to improve the ionization resistance of the blend. It is taught that adding silicone gums to such polymers has an adverse effect on the mixing characteristics.
Falender et al. in U.S. Pat. No. 3,865,897 teach a method of combining certain polyolefins and high viscosity silicone gums that overcomes the adverse mixing characteristics and gives an improved blend. They teach that when polyethylene or ethylene vinylacetate copolymer containing up to 10 weight percent vinylacetate and a polydiorganosiloxane gum containing from 1.5 to 17 mol percent of siloxane units having vinyl or allyl groups are mechanically mixed under certain conditions of shear and temperature, a graft copolymer is formed which results in an improved blend. Falender et al. further teach that employing polydiorganosiloxane gums which contain vinyl and allyl contents outside the above stated limits produces inferior products, e.g. poor blends which tend to separate even after processing. While the blends taught by Falender et al. possess many advantageous and useful properties, commercial utilization of the blends has been slow because of the high vinyl content required in the polydiorganosiloxane gum. The alkylvinylsiloxane units of the siloxane are more difficult to obtain, have a more limited availability and cost more than the dialkylsiloxane units. This limits the commercial use of the blends of Falender et al. to relatively few areas.
It is one of the purposes of the present invention then to provide a method of obtaining an improved blend of a low vinyl or allyl content polydiorganosiloxane gum and an ethylene vinylacetate copolymer. Another purpose of the present invention is to provide a method of mechanically mixing a low vinyl or allyl content polydiorganosiloxane gum and an ethylene vinylacetate copolymer to obtain an improved blend containing graft copolymer. These purposes and others will be apparent from the detailed description of the invention.